Research has shown that school-based drug abuse prevention programs are able to prevent substance misuse. The mechanisms through which prevention programs impact substance misuse, however, remain generally unclear. While it is likely that factors related to the transition from middle school to high school place high school youth at risk for substance misuse, few studies have invested prevention programs developed for high school youth and their intervening processes. Moreover, although a few investigations suggest that prevention programs impact Latino youth via similar processes as Black and White youth, there is some evidence to suggest that there are differences in how prevention programs influence Latino youth. In particular, research suggest that level of acculturation may play an important role in how these youth respond to prevention programs, especially when gender is considered. To broaden our understanding of the intervening mechanisms of high school-based prevention programs and how they influence Latino youth, the proposed study will examine whether increased competencies and reduced risk factors predict reduced growth in substance use in a Latino sample of high school students. This study aims to: 1) examine the impact of a school-based ninth grade prevention program, Peer Group Connection (PGC), on students'alcohol and marijuana use over three years, 2) examine disapproval of peers'alcohol or marijuana use, beliefs about alcohol or marijuana use, personal and social competencies, and school bonding as potential mediators of program effects, and 3) examine gender and acculturation as potential moderators of program effects. In 2005, 269 ninth grade students (92% Latino) were randomized to the control (n = 175) and program (n = 94) conditions of a randomized controlled trial examining the effects of PGC. Under the supervision of trained advisors, trained upperclassmen delivered PGC's year-long, activity-based curriculum to small groups of freshmen during weekly forty-minute sessions. Four waves of survey data were collected: baseline, post-test, one-year follow-up, and two-year follow-up. Data will be analyzed using latent growth curve analysis to model individual change in the potential outcomes and mediators. An improved understanding of how prevention programs contribute to reductions in the incidence and progression of Latino youth's substance abuse may have implications for future preventative approaches. Thus, this study is in line with NIDA's goal "to prevent the initiation of drug use and the escalation to addiction in those who have already initiated use."